Electric drives for two-wheeled vehicles, for example E-bike motors, and electronic bicycle tachometers normally require sensors for detecting the wheel or pedal rotations in order to determine, inter alia, the speed therefrom. These self-evidently include signal-generating devices for such sensors.
Normally, in systems of said type, magnets are provided as pulse generators (signal generators) for sensors in the form of reed contacts, Hall sensors or similar sensors. Every time the magnet passes the sensor, the tachometer, for example, receives an item of signal information, and from this calculates speed, distance, average speed, section distance etc. In the case of the E-bike with central motor, it is the case here for example in Europe that, with such systems, motor assistance is provided up to a limit of 25 km/h. In the USA, the maximum km/h value for assistance in the case of E-bikes is 32 km/h.
Normally, the signal-generating magnet is attached to a spoke in the front and/or rear wheel. This magnet is highly visible and adversely changes the design. The magnet can likewise twist, resulting in a loss of function. A spoke magnet of said type can, through negligence, easily twist or become lost, thus bringing the tacho and/or the entire E-bike drive system to a complete standstill. In the case of racing bikes, the spoke magnet generates a considerable increase in air resistance, and thus an additional disadvantage.
To solve the problem, it has already been considered to incorporate the magnet into the rubber of the tread or flank of a tire. In this regard, WO 2004/057274 A1 discloses a revolution counter for a bicycle, in the case of which a magnetic element is incorporated, in the plane of symmetry of a wheel, within the tire or within a hose situated in the tire. During every revolution of the wheel, the magnetic element moves through the capture area of a sensor which is fastened to the bicycle frame and which responds to magnetic induction, such that the rotations can be counted. The magnetic element may be adhesively bonded to the inner side of the tire or hose. Here, it is disadvantageously necessary for relatively thick magnets to be used in order to ensure sufficient permeability through the rubber layers of the tire or hose and generate a signal at the sensor.
If the magnet is enlarged to the point that an acceptable range is attained, the problem arises that, owing to the size, it becomes difficult for the magnet to be incorporated in the tire. A further problem is that the magnet is a foreign body in the rubber, and is sooner or later pushed through the surface as a result of the flexing work of the tire. Adequate functioning is thus lost. Furthermore, the tire may be damaged and become unusable. Functional tests with prototypes of various incorporated magnets have confirmed this.
DE 3911627A1 discloses a lighting system for bicycles, in the case of which permanent magnets are integrated in a wheel of the bicycle. In one embodiment, said permanent magnets are attached to the inner side of the tread of a tire and interact with a generator which is fastened to the frame of the bicycle. Here, there is the disadvantage that, during the introduction of permanent magnets into the rubber material of the tire, that is to say during the vulcanization, the magnetic characteristics can be adversely affected.
The heat required for the vulcanization of the tire (approximately 180° C. over 200 seconds) can reduce the energy value of the incorporated magnets, because magnets lose their magnetic action under intense heat. Magnets exist which can withstand such temperatures, but these have energy values too low to ensure reliable functioning.
DE 10 2010 042808 A1 discloses another solution, specifically a device for rotational speed detection having a magnet piece arranged on a valve of a hose, the trajectory of which magnet piece runs through the measurement region of a sensor attached to the frame. A disadvantage here, however, is the fact that the valve is seated centrally in the wheel rim and is thus relatively far remote from a sensor, which is for example in the form of a reed contact.